In recognition of their inherently cleaner and potentially more efficient fuel burning properties, fluidized bed boilers are now seriously being considered as viable supplements to the traditional pulverized coal and stoker fired vapor generating units of today.
Briefly, a fluidized bed boiler burns granulated coal in a floating fluid-like suspension called a fluidized bed. In addition to the coal, a sorbent (usually limestone) is introduced into the bed to absorb a portion of the noxious gases generated as a result of the burning process. By introducing fluidizing air from beneath the burning zone through an air distribution plate or through the furnace floor, the burning coal actually floats above the plate or the floor on a cushion of air as it is consumed. As a result of the enhanced combustion process, greater quantities of heat may be generated. And, as a consequence of the introduction of the sorbent, undesirable pollution levels are substantially reduced.
As a result of the fluidized bed design, it is necessary to distribute the coal-limestone mixture in a uniform manner over the entire cross-sectional area of the bed. Present technology requires that there be one fuel distribution point for each nine square feet of bed area. This means that a boiler rated at a modest 80,000 pounds-steam per hour would require approximately twenty such points. As the rated capacity of the boiler increases, the number of necessary feed points will correspondingly increase.
Some of the present day fluidized bed designs utilize peripheral wall mounted feed nozzles to distribute the fuel and/or sorbent mixture into the bed. As the size of the boiler increases, it may become very difficult to introduce an even fuel distribution over the entire bed surface from wall mounted nozzles.
Other concepts include positioning the feedpipes and/or the fuel nozzles within the bed above the plate or above the furnace floor. Unfortunately, these designs may lead to feeder component erosion, overheating and plugging. Furthermore, the replacement of such units may prove to be difficult as well.
Clearly, in improved fuel feeder design is desirable.